From BlenderWiki

Focal Length Constraint Tests

The following table gives the results of bundle refinement on the footage provided with Andrew Price's camera tracking tutorial. He suggests in the tutorial that it was filmed with a focal length of 18mm.

	Test 1	Test 2	Test 3
Initial F	18.000mm	25.775mm	30.000mm
Unconstrained	25.775mm	25.775mm	25.849mm
Large range	25.826mm	25.775mm	25.846mm
Initial F ± 1mm	19.000mm	25.775mm	28.990mm
Initial F ± 5mm	23.000mm	25.775mm	25.909mm
Initial F ± 10mm	25.792mm	25.775mm	25.840mm

Test 1 shows that when given an initial focal length of 18mm (as suggested in he tutorial), the refined focal length is approximately 25.8mm unless the constraint range does not include this value, in which case the refined focal length is the value in the range closest to this value. In test 2, when given 25.775mm as the initial focal length, it is left unaffected under refinement. Test 3 shows similar results to the first test.

This almost suggests that doing the constrained refinement is unnecessary - we could simply do unconstrained refinement (which appears to be faster), and then choose the value closest to refined focal length in the given constraint range. However, this is probably only the case with this footage, so it would be good to test on footage that requires the constraint to get an accurate refinement.